The nature of the Pt-H bonding for strongly and weakly bonded hydrogen on platinum. A XAFS spectroscopy study of the Pt-H antibonding shaperesonance and Pt-H EXAFS

A long-standing problem in the research of Hz chemisorption on supported Pt catalysts is the unclarity about the difference in nature of the Pt-H bond ing for weakly and strongly bonded hydrogen. On the basis of Pt-Pt EXAFS an d a newly developed XANES analysis of the Pt L-2 and L-3 edges, it will be shown that both types of hydrogen are truly chemisorbed species. The EXAFS analysis shows a similar contraction of the Pt-Pt bond irrespective of the removal of weakly or strongly bound hydrogen, indicating a similar nature f or both interactions. Moreover, the analysis of the L-2 and L-3 X-ray absor ption edges shows unambiguously the presence of a Pt-H antibonding state fo r both the weakly and strongly bonded hydrogen. This is a clear indication for chemisorption of hydrogen for both types of hydrogen. The Pt-H EXAFS sh ows a smaller Pt-H bond length in the case of strongly adsorbed, which is i n correlation with the expected strength of the Pt-H bond.